Hollow, acrylate polymer microspheres are known in the art for a variety of applications. They may be tacky or tack-free. Tacky, hollow, acrylate polymer microspheres are especially useful for providing repositionable pressure sensitive adhesives. The term "repositionable" refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability. Tack-free hollow acrylate polymer microspheres, on the other hand, are especially useful as fillers for products such as sealers, coatings and adhesives. They can also be useful as dry lubricants, vibration damping materials, and rheology control agents.
U.S. Pat. No. 5,053,436 (Delgado), assigned to the assignee of the present case, describes tacky, hollow microspheres that are useful as repositionable pressure sensitive adhesives. The microspheres comprise at least about 85 parts by weight of at least one alkyl acrylate or methacrylate ester and up to 15 parts by weight of at least one polar monomer. Two methods are described for preparing aqueous suspensions of the tacky, hollow microspheres. The first method employs a "two-step" emulsification process comprising the steps of (1) forming a water-in-oil emulsion of a water phase (comprising water and at least one polar monomer) in oil phase monomer(s) using an emulsifier; and (2) forming a water-in-oil-in-water emulsion by dispersing the water-in-oil emulsion into an aqueous phase comprising an emulsifier. An aqueous suspension of monomer droplets is formed which upon polymerization become microspheres. A majority of the microspheres have at least one interior cavity that upon drying becomes an interior void.
The second method described for preparing the hollow, tacky microspheres employs a "one-step" emulsification process comprising the step of forming droplets by mixing oil phase monomer, moderately ionized polar copolymerizable monomer and an emulsifier. Here again, an aqueous suspension of monomer droplets is formed which upon polymerization become microspheres.
WO 93/12147 (Delgado), also assigned to the assignee of the present case, describes tack-free, hollow, acrylate microspheres that are useful as fillers. This reference also describes "two-step" and "one-step" emulsification processes for preparing aqueous suspensions of microspheres. The "two-step" emulsification process of this reference comprises the steps of (1) forming a water-in-oil emulsion of a water phase (comprising water and optionally at least one polar monomer) in an oil phase (comprising oil phase monomer(s), at least one multifunctional crosslinking agent, and at least one oil-soluble free radical initiator) using an emulsifier and (2) forming a water-in-oil-in-water emulsion by dispersing the water-in-oil emulsion into an aqueous phase comprising an emulsifier.
The "one-step" emulsification process of WO 93/12147 comprises the step of forming droplets by mixing oil phase monomer, at least one polar copolymerizable monomer, at least one multifunctional crosslinking agent, at least one emulsifier, at least one oil-soluble free radical initiator, and an aqueous phase.
Of the two methods described above for preparing tacky and tack-free hollow, acrylate polymer microspheres, the "one-step" emulsification process is preferred. This is because the one-step method is more efficient and cost effective than the two-step method, requiring less steps to produce an emulsion. The "one-step" emulsification process, however, requires the use of a polar copolymerizable monomer. Thus, it would be desirable to provide a "one-step" emulsification process that can be used without polar copolymerizable monomer to provide hollow, acrylate polymer microspheres that are either tacky or tack-free.